That's a very theoretical view. (As most absolutes are.)

Unions and rules around unions can be very different depending on locality, industry and other specifics. The power and benefits a union gives a specific employee may not outweigh the cost they impose on that specific employee.

Furthermore, unions are organizations. They have their own internal power structures that can be corrupted by self-serving individuals or special interests. A blanket "union = good" view can make that invisible to you.